(1) A novel magnesium-dependent, calcium-inhibited, phosphoseryl protein phosphatase from bovine brain has been purified to near homogeneity and partially characterized. It is a 540,000 molecular weight protein consisting of six subunits. Its activity is enhanced by high enzyme concentration, interaction with a 78,000 molecular weight phosphatase, and by the presence of polyamines. (2) Cytosolic calcium oscillations in three types of cells have been investigated by following the time course of vasopressin-induced production of inositol trisphosphate. The results suggest that the frequency of calcium spiking correlates with the steady-state level of inositol trisphosphate. (3) The role of the carboxy terminal end of the R2 subunit of Escherichia coli ribonucleotide reductase in the electron transfer between R1 and R2 proteins has been studied by site-directed mutagenesis. A conserved residue in the carboxy terminal region, tyrosine 356, seems to serve as an intermediary in the electron transfer process.